52 Project 2012: Foundational Electronic Components #7 – LEDs

What is an LED?

A light-emitting diode (or “LED” for short) is a special kind of diode which converts some electrical energy into a particular wavelength of light.

How does an LED work?

Since we’ve already discovered how regular diodes work (using two oppositely doped semiconductor material layers back-to-back), not much extra explanation is necessary to discover the secrets of LEDs. Electrically, they function the same way that regular diodes do. They only allow current to flow in one direction, and they have a forward voltage drop and maximum power dissipation.

The flow of energy through a circuit has the potential to generate visible light at any time, based on the physical properties of the conducting (or semiconducting) material. Actually, one of the byproducts of the movement of electrons from one atom to another is that this tiny change in energy at an atomic level releases photons—the basic element of light. This actually happens all the time, but most of the time, we cannot see it. All diodes could therefore be called “light-emitting” diodes, but the light emitted by them is usually outside the visible light spectrum.

LEDs, on the other hand, are manufactured with a very specific elemental composition such that the exact change in energy due to the movement of electrons generates a very specific visible wavelength of light, such as red, green, blue, orange, etc. With the right kind of material, just about any desired kind of light can be generated. Most remote controls use an infrared LED to send different commands. This is again the exact same technology, just using a type of light we cannot see with the naked eye.

Due to advancing technology and a consistent drop in the cost of semiconductor material used to manufacture LEDs, today they are rapidly replacing traditional incandescent or fluorescent lighting in many areas. They are far more efficient and durable, and can deliver more light in a much smaller package. LEDs are one of the main reasons that TV displays, computer monitors, and smartphone screens have become so thin over the last decade.

LED ratings

Since LEDs are electrically the same as regular diodes, they have the same forward voltage drop and peak inverse voltage ratings.

Another new rating for LEDs is the wavelength of light generated, usually expressed in nanometers (nm). The visible light spectrum for humans is between about 390nm and 750nm. Typical red LEDs emit somewhere around 650nm light. Full-color computer and TV displays use thousands or millions of individual red, green, and blue LEDs and the technique of additive color to create the perception of any imaginable color at any point on the display.

Due to their added function of converting energy to light and the nature of the special semiconductor material, LEDs also have an important maximum current rating. Typical small LEDs should not be run individually at more than about 15-20 milliamps, because a larger current will generate too much heat in the semiconducting material, causing it to burn up. This doesn’t usually create a fire hazard, but it permanently damages the LED.